Larry “Major Nelson” Hyrb announced the opening of Microsoft accepting applications to join the latest Beta testing phase of the next Xbox Live Update on his blog. The Beta includes updates to improve overall performance and the ability to conduct transactions with local currency. The latter item being due to the fact Microsoft Points are being phased out and real money transactions taking over towards the end of the year. Invites go out to all countries covered by Xbox Live except for Japan which, Major Nelson mentions, will be invited at a later date.
You can apply to join the Beta by looking for the sign up option on the Home screen of the Xbox 360 Dashboard the next time you load up your console. Signing up requires you to agree to a non-disclosure agreement (NDA) for confidentiality reasons. A Beta FAQ will be available later in the day at http://support-preview.xbox.com/xbox-360/system-updates/xbox-360-beta-faq *Not currently live at the time of writing*.